After the occurrence of a double-strand break in DNA, the chromatin needs to be relaxed to allow DNA repair, either by HRR or by NHEJ. There are two pathways that result in chromatin relaxation, one initiated by PARP1 and one initiated by γH2AX . Chromatin remodeling initiated by γH2AX depends on RNF8, as described below. The histone variant H2AX constitutes about 10% of the H2A histones in human chromatin. At the site of a DNA double-strand break, the extent of chromatin with phosphorylated γH2AX is about two million base pairs. γH2AX does not, by itself, cause chromatin decondensation, but within seconds of irradiation the protein “Mediator of the DNA damage checkpoint 1” specifically attaches to γH2AX. This is accompanied by simultaneous accumulation of RNF8 protein and the DNA repair protein NBS1 which bind to MDC1. RNF8 mediates extensive chromatin decondensation through its subsequent interaction with CHD4 protein, a component of the nucleosome remodeling and deacetylase complex NuRD.
DNA end resection is a pivotal step in HRR repair that produces 3’ overhangs that provide a platform to recruit proteins involved in HRR repair. The MRN complex, consisting ofMre11, Rad50 and NBS1, carries out the initial steps of this end resection. RNF8 ubiquitinates NBS1, and this ubiquitination is required for effective homologous recombinational repair. Ubiquitination of NBS1 by RNF8 is, however, not required for the role of NBS1 in another DNA repair process, the error-prone microhomology-mediated end joining DNA repair. RNF8 appears to have other roles in HRR as well. RNF8, acting as a ubiquitin ligase, mono-ubiquitinates γH2AX to tether DNA repair molecules at DNA lesions. In particular, RNF8 activity is required to recruit BRCA1 for homologous recombination repair.
is a dimeric protein complex, a heterodimer of two polypeptides, Ku70 and Ku80. Ku protein forms a ring structure. An early step in non-homologous end joining DNA repair of a double-strand break is the slipping of a Ku protein over each end of the broken DNA. The two Ku proteins, one on each broken end, bind to each other and form a bridge. This protects the DNA ends and forms a platform for further DNA repair enzymes to operate. After the broken ends are rejoined, the two Ku proteins still encircle the now intact DNA and can no longer slip off an end. The Ku proteins must be removed or they cause loss of cell viability. The removal of Ku protein is performed either by RNF8 ubiquitination of Ku80, allowing it to be released from the Ku protein ring, or else by NEDD8 promoted ubiquitination of Ku protein, causing its release from DNA.
-induced formation of pyrimidine dimers in DNA can lead to cell death unless the lesions are repaired. Most repair of these lesions is by nucleotide excision repair. After UV-irradiation, RNF8 is recruited to sites of UV-induced DNA damage and ubiquitinates chromatin component histone H2A. These responses provide partial protection against UV irradiation.
Impaired spermatogenesis
is the process in which spermatozoa are produced from spermatogonial stem cells by way of mitosis and meiosis. A major function of meiosis is homologous recombinational repair of this germline DNA. RNF8 plays an essential role in signaling the presence of DNA double-strand breaks. Male mice with a gene knockout for RNF8 have impaired spermatogenesis, apparently due to a defect in homologous recombinational repair.
